Optical scanners of a type which are used in laser printers generally include a rotatable polygon mirror which is used to scan a light beam across a receiving medium. The scan optical elements used in such scanners are designed to achieve a flat tangential field for good beam focus and to correct for so-called pyramidal errors, that is, spot position errors in the cross-scan direction resulting from angular misalignment of the facets on the polygon; the optical elements must also produce a relatively straight scan line and correct for the F-.theta. distortion condition. The input optics for such scanners, which typically consist of a laser, collimating and beam shaping optics, and noise reduction and laser modulation means, precondition the incoming beam to the polygon mirror. These optics control various beam parameters, including the size, shape, and wavefront quality, as well as controlling the noise and various thermal effects. The receiving medium in the scanners can be a photographic film or a photosensitive medium such as a photoconductive drum.
U.S. Pat. Nos. 4,921,320, 5,151,810 and 5,237,348 illustrate the basic configuration of the input optics for a laser printer, consisting of a laser source, beam shaping optics, a rotating polygon mirror, and beam scan optics. As is typical, the input optics to the polygon mirror consist of a diode laser, a collimator lens, a beam expander, and a beam shaping system. These beam shaping optics usually comprise a system of spherical lenses, mirrors, positive cylinder lenses, and negative cylinder lenses. Recent literature discloses considerable effort in improvements to the design of the post-polygon scan optics. Whereas, the principal features of the prepolygon input optics have generally changed less and are described by earlier patents.
U.S. Pat. No. 4,203,652 discloses a multiplicity of means for shaping the beam from the diode laser to compensate for the different angles of divergence and different sources of divergence in the orthogonal directions. These systems comprise various combinations of spherical lenses and crossed cylinder lenses for collimating and beam expanding the beam to match the design parameters.
U.S. Pat. No. 4,253,724 describes some of the other optical components typical to the pre-polygon optical path for laser printers. This patent describes the input optical means for an IR (Infra Red) based laser printer, which includes direct laser modulation of a single mode semi-conductor laser, a variety of laser collimating means, an anamorphic afocal beam expander lens system for correcting diode laser astigmatism, and a limiting anamorphic beam shaping aperture.
In such prior art systems, the common variations typical among standard manufactured diode lasers can cause problems. Typically, the laser beam divergence in both the minor and major axis directions can vary by +/-20%. Without corrective action, these variations will cause the spot size at the media to change in size from printer to printer. Within the limits of the pre-polygon optical design, these variations can be corrected for by adjusting an anamorphic beam expander lens system, but at added cost and complexity to the manufacturing process. Alternatively, the beam size can be corrected by apodizing or truncating the beam, though this reduces the available optical power and requires correction in the lens design to compensate for beam truncation effects on the spot size. Finally, the manufacturer, at some cost, can specify delivery only of laser diodes that are tested to meet some predetermined divergence specification.
While the prior art discloses the preconditioning of the beam input to the polygon mirror to adapt to the static features typical of diode lasers, there has also been disclosed a variety of methods for controlling the dynamic properties typical to semiconductor lasers. The features of the emitting area of diode lasers are typically quite small, on the order of a few microns. Often, the laser surface, or front facet, is imaged to the recording media at high magnification. At such high magnifications, small shifts in the position of the laser relative to the collimating lens can cause the image pixel focus to shift at the medium. These shifts can cause significant changes in the image pixel size and shape, which appear as undesirable artifacts in the printed image. As disclosed in U.S. Pat. No. 4,948,221, a common practice to alleviate this problem is to design an athermal head for the laser diode which keeps the laser at the focal point of the collimator as the laser changes. This is done by using a combination of dissimilar materials so that when the temperature changes, the laser position is effectively maintained with respect to the collimator. Thus, the object distance of the optical system does not change. This approach can become very difficult and expensive when the tolerances are tight.
European Patent Application 0 323 850, published Jul. 12, 1989, discloses another method to actively compensate for image pixel motion by sensing its motion and actively adjusting a lens position so as to compensate for the motion. This method does add complexity and cost, but it compensates for thermal changes throughout the entire optical system, rather than for just those parts near the laser.
Once a laser printer is assembled, including the laser-collimator assembly, the pre-polygon beam shaping optics, and the post-polygon scan optics, the laser printer is vulnerable to laser failure. As a result of the extensive and demanding alignment and beam quality specifications common to laser printers, such printers are not typically field serviceable. As laser printers are designed increasingly for more demanding, higher power, higher resolution applications, the field failure of a laser is becoming increasingly costly.
The prior art describes both mechanisms for preconditioning the beam from the laser for compatibility with the polygon and the post-polygon scan optics and mechanisms for minimizing image artifacts caused by thermal variations within the laser package. Yet there is still a need for a relatively simple input optics system for a laser printer in which both the effects of laser variability and thermal expansion and contraction within the laser-collimator assembly are minimized. There is also a need for improving the serviceability of the entire laser printer system to laser failure over prior methods.
It is an object of the present invention to overcome the problems in the prior art discussed above, and to provide an improved laser printer/optical scanner.